Gary Z

With all the discussion of true duals and dumped bullets I wondered whether my single tailpipe might be costing me power. I have made careful measurements of my exhaust system backpressure. My basic discovery is that the peak pressure in my I-pipe is approximately 2 psi above atmospheric pressure. My guess is that this pressure is low enough to not cause significant power loss.

My NA 347 made 412 rwhp on a Mustang dyno. My exhaust system consists of QTP longtube headers, a modified SLP off-road Y-pipe, the original F-body intermediate pipe, and a 3-inch Magnaflow single-in, single-out muffler. The inside diameter of the stock I-pipe is only 2 3/4 –inch. My main concern was that this pipe was too restrictive. I already had an unused oxygen-sensor bung in the I-pipe so this was my measuring point. I used an ACDelco 2-Bar MAP Sensor (GM 12615136) to measure pressure. I recorded the sensor output using HPTuners Scanner.

The attached graph shows WOT exhaust pressure recorded during a recent dragstrip pass. Although the data is corrupted by turbulence, the pressure response to throttle position is clearly visible, with pressure falling to 100 kPa during shifts and rising to 115 kPa at WOT.

These measurements make me think that I don’t need duals or a larger (3-inch) I-pipe.

JL ws-6

iTrader: (50)

I would dump everything from the collectors back to be honest.

You want a full 3 inch Y pipe that runs into a good merge type collector, and at least a full 3 inch pipe to the back to whatever muffler/catback you want (other then the stock one and flowmaster as they're both junk, if you are running a regular muffler sideways and don't mind the single tip your options are wide open)

I bet there's a good 10 hp across the board with what you have on the car compared to a larger diameter system.

I went with a set of 1 7/8 stepped to 2 inch headers with 3.5 collectors, and on each side ran a 3.5 inch pipe to a super 44 muffler, welded the 3.5 inch pipe to the case, and ran enough 3.5 inch pipe to get the exhaust to exit where I wanted it to.

No x or h as I don't believe in that, run a big enough pipe and that is a waste IMO. Granted I have a motor plate so I was able to tuck the exhaust right on the floor, I don't street drive the car but it is quiet enough so I probably could. Sounds good and I can drop the system off the car and it makes no difference in performance at all. Car goes 9.5x regardless, soon to be a good bit quicker.

ATLSS

iTrader: (7)

Scan or not, you're losing power with that setup.

I agree with JL.

This is the reason I went with the ARH headers and 3" y-pipe that merges into a 3.5"... then my catback is a Mufflex 3.5" with a single Magnaflow 3.5" in/out muffler.... The sound is awesome and the performance is no different than open exhaust (headers only)...

JL ws-6

iTrader: (50)

Right. Now, run 2 3.5 inch setups.. that's what I put on my car. Enogh flow for bottom 8's probably before it's a restriction, but better off without the restriction then with.

Gary Z

Thanks for the suggestions. If dumped bullets off the headers would give me 10 hp, I would jump at that. I have no doubt that my current system would be inadequate if I were in the nines contemplating eights. But then I would have a bigger motor pumping much more air. My little motor doesn’t need as much. The best way to know would be to run (dyno or strip) with open headers. That is not an option around here. These considerations lead me to think it might be worthwhile to measure my backpressure. I expected to find higher pressures. Two psi seems borderline.

-Gary

JL ws-6

iTrader: (50)

Tune the car in open loop, so the o2's are all off, go to the track and make a pass, then drop the exhaust off the car and put enough weight on teh floor behind the seats so there's no change in raceweight and make another pass.


That will tell you if you have a restriction or not. I can tell you, getting a 4 inch catback and a 3 into 4 inch Y pipe will pick up alot, bullet's off the headers or a magnaflow oval muffler would work as well, they have some small ones that will work.